1. Field of the Invention
The present invention relates to a fluid heater and a fluid heating apparatus for a high purity liquid such as ultrapure water, various chemical liquids, and the like, and more particularly to a fluid heater which is suitably useful in a pipe or the like for a fluid that is handled in, for example, an apparatus for producing a semiconductor device, liquid crystal display devices, or chemicals, and a production line for foods, and a fluid heating apparatus which is configured by combining a plurality of such fluid heaters.
2. Explanation of Related Art
Some of fluid heaters of this kind are configured so that a heater is disposed in a casing with being passed through the casing, and a fluid passing through the interior of the casing is heatable by the heater. For example, the outer face of a metal-made rod-like heater is covered by a metal material with reduced metal elution such as special stainless steel, and the heater is passed through a cylindrical casing which is made of a clean synthetic resin material, and both ends of which are closed. A fluid is caused to flow through the casing with using a fluid inlet and a fluid outlet which are disposed in the end portions of the casing, respectively. Therefore, the fluid passing through the interior of the casing or staying therein can be heated by the rod-like heater.
The casing constituting the above-mentioned fluid heater is strictly requested not to leak the fluid. Namely, in addition to prevention of leakage from the inlet and the outlet for the fluid passing through the interior, leakage prevention is applied also to the passing position of the rod-like heater in the casing, and therefore the number of places where the countermeasure for preventing leakage is to be taken is large. In a fluid heater in which a rod-like heater is penetratingly disposed, it is an issue to form a leakage-free casing in an excellent productivity state. It is contemplated that the configuration disclosed in Patent Reference 1 is employed in the casing.
Japanese Patent Application Laying-Open No. 10-160362 discloses a heat exchanger in which heat exchange tubes are passed into a cylindrical casing. The structure of the heat exchanger will be described. As shown in FIGS. 21 and 22, in order to ensure the casing 81 through which the heat exchange tubes 80 pass to have a sufficient sealing property that can withstand an internal pressure of a certain degree, plural metal fastening members 83 such as tie rods or through-bolts are placed in parallel on the outer periphery of a shell 82 constituting the body of the casing 81 so as elongate along the longitudinal direction. Both end portions of the metal fastening members 83 are passed through lid members 84 placed in both end portions of the shell 82. Nuts 85 are fastened to external thread portions in both ends of the metal fastening members 83 protruding from the lid members 84, respectively, whereby hermetical seal is provided between butting faces of the end portions of the shell 82 and the lid members 84. As a result, the casing 81 is hermetically configured. Furthermore, O-rings 86 serving as sealing members are interposed between the butting faces of the end portions of the shell 82 and the lid members 84.
In the structure in which the end portions of the shell 82 and the lid members 84 are sealed by fastening the metal fastening members 83 such as tie rods or through-bolts with the nuts 85, however, the number of components for sealing is so large that the cost is increased and the casing structure is enlarged. When the heat exchanger is installed in a place where it is exposed to a sulfuric acid ambient or the like, the metal fastening members 83 easily corrode, and metal contamination inevitably occurs. Recently, therefore, restriction of use of such a heat exchanger is highly requested particularly in the field of semiconductor devices.
In order to avoid loosening of the metal fastening members 83, the metal fastening members 83 must be periodically refastened. Usually, the metal fastening members 83 are plural or at least four, and hence the degrees of refastening of the metal fastening members 83 are easily dispersed. This dispersion produces the possibility that the lid members 84 and the shell 82 are deformed. When the lid members 84 and the shell 82 are deformed, twisting or distortion is produced between the end portions of the shell 82 and the lid members 84, and hence there arises a problem in that local stress concentration occurs to promote development of creep. The center axes of metal tie rods of the metal fastening members 83 fail to coincide with those of metal tie rod sheaths, thereby causing problems in that both the members rub against each other to increase the sliding resistance, and that generation of abrasion dust containing metal powder is caused. When the shell 82 and the lid members 84 are deformed, moreover, these members must be replaced with new ones. Usually, the members are machined products, and hence relatively expensive. In the structure, therefore, recycling in which the casing structure is replaced with new one and the internal devices (heat exchange tubes 80) remain to be used is hardly performed.
In the case where the connecting structure in which the O-rings 86 serving as sealing members are interposed between the butting faces of the end portions of the shell 82 and the lid members 84 is applied to a fluid heater, the use of the O-rings 86 causes the corrosion resistance and the range of the service temperature to be limited. For example, a high-temperature chemical liquid cannot be passed through the spaces surrounding the O-rings 86. Furthermore, contamination due to dust generation of the O-rings 86 may be problematic. Recently, therefore, restriction of use of a component such as the O-rings 86 is highly requested in the field of semiconductor devices.
In the case where a fluid heater of this kind is to be used in heating a chemical liquid or the like, a fluororesin having excellent corrosion resistance, such as PTFE or PFA is often used in the components such as the shell 82 and the lid members 84. However, a fluororesin has high lubricity, and hence creep due to vibration or heat of a pipe occurs in the connecting portions between the shell 82 and the lid members 84. As a result, there arises a problem in that the metal fastening members 83 such as tie rods or through-bolts are loosened and fluid leakage from the connecting portions in the ends of the shell 82 is caused.
Alternatively, thread sealing or welding may be employed as the casing connecting structure between the shell 82 and the lid members 84. However, these measures are not highly effective. In a sealing structure which is based simply on threads, a high sealing property cannot be obtained, the pressure resistance is not sufficiently high, and leakage due to creep easily occurs. Usually, welding requires a skilled technique, and cannot be conducted by an easy work. Therefore, welding has problems in that the production efficiency is low, that the on-site workability is poor, and that it is difficult to conduct maintenance and inspection on the site. Furthermore, welding has aspects in which, even when specification is changed, replacement of internal components such as a heater is substantially impossible, and the components are not suitable to recycle or reuse.